The present invention relates to a waste treatment system, and more particularly to a waste treatment system configured for treatment of cyanide-containing waste by high temperature hydrolysis.
A wide variety of industrial operations, such as steel manufacturing, plating operations, petrochemical operations, and aluminum smelting, use cyanides and/or create cyanides. Liquid and solid wastes from these operations may well be contaminated with alkali metal cyanides, silver cyanides, or cyanides in a wide variety of other forms. Where the wastes have been improperly stored at a hazardous waste dump, it is also possible that the soil at the dump site will become contaminated with cyanides. As a result, it has become necessary to develop processes capable of treating waste liquids, waste sludges, and slurries made from contaminated soil to remove or destroy substantial portions of the cyanide contaminants.
A number of researchers have focused on the use of chemical oxidation to detoxify cyanide-containing wastes. For example, alkaline chlorination has been used. However, this process may not be effective to destroy some complex cyanides. Sulfur dioxide in the presence of air and a copper catalyst has also been used to oxidize cyanides.
It has been reported that cyanides can be hydrolyzed to form ammonia and formic acid, both of which can readily be disposed of in an environmentally sound manner. See, for example, U.S. Pat. No. 4,042,502. The hydrolysis reaction is as follows: EQU CN.sup.- +2H.sub.2 O.fwdarw.NH.sub.3 +HCOO.sup.-.
In order for this reaction to take place at rates sufficient for industrial purposes, it is necessary to subject the cyanide-containing waste to both high temperatures and pressures. Temperatures of 120.degree.-200.degree. C. (248.degree.-392.degree. F.) are commonly employed, along with pressures from about 5 to about 100 atm.
Catalysts, while not essential, have also been used. For example, U.S. Pat. No. 3,945,919 discloses the use of various transition metal catalysts in conversion of cyanide-containing waste by hydrolysis.
One major problem with designing systems to carry out cyanide hydrolysis on an industrial scale is that the hydrolysis reaction results in the formation of solids which tend to aggregate in the system as the reaction progresses. The solids can clog pipes and hamper valve operation, but perhaps more importantly can collect on heat transfer surfaces, significantly impairing heat transfer and thus limiting system efficiency.
One attempt to deal with the clogging problem in cyanide hydrolysis systems is disclosed in U.S. Pat. No. 4,877,519. There, a first double pipe heat exchanger is provided for an influent waste stream to a hydrolysis reactor and a second double pipe heat exchanger is provided for an effluent stream from the reactor. The influent and effluent streams pass through the tube side of the first and second heat exchangers, an arrangement which is said to minimize clogging of the heat exchangers. The reactor is designed to provide intermittent discharge of effluent for brief intervals, which is also said to avoid solids build-up in the system.
However, even in systems of this type, the influent and effluent waste streams must flow through a piping network, and clogging may occur. In addition, because the waste influent is preheated in a first heat exchanger, solids deposition may actually begin to occur in the first heat exchanger, and the heat exchange surfaces may become fouled. It would be desirable to provide a system for cyanide destruction by high temperature hydrolysis in which the potential for system problems due to solids build-up is minimized by avoiding the need to flow the cyanide-contaminated waste influent and the substantially detoxified waste effluent through an extensive network of piping and valves. It would also be desirable to provide a system in which the cyanide-containing waste is heated to a temperature sufficient to provide hydrolysis and is thereafter cooled to a safe discharge temperature in a single vessel without preheating the cyanide-containing waste influent.
According to the present invention, a system is provided for treatment of cyanide-containing waste to form a substantially detoxified reaction product. The system comprises a reaction vessel, a heat exchange conduit extending through the reaction vessel, a heating loop and a cooling loop both serving the heat exchange conduit, and means for selectively diverting the flow of heat exchange fluid between the heating loop and the cooling loop. The heat exchange conduit includes an inlet and an outlet. The heating loop includes a heating vessel, a heating loop delivery conduit extending between the heating vessel and the inlet, and a heating loop recycle conduit extending between the outlet and the heating vessel, a cooling loop, a cooling loop delivery conduit extending from the cooling apparatus to the inlet, and a cooling loop recycle conduit extending from the outlet to the cooling apparatus. The diverting means diverts the flow of heat exchange fluid at the outlet between the heating loop recycle conduit and the cooling loop inlet conduit. Thus, heat exchange fluid flows between the heating loop and the heat exchange conduit for a time sufficient to convert the cyanide-containing waste in the reaction vessel into the substantially detoxified reaction product. Thereafter, the heat exchange fluid flows between the cooling loop and the heat exchange conduit for a time sufficient to cool the substantially detoxified reaction product to a safe discharge temperature.
The invention further comprises a process for treating cyanide-containing waste to form a substantially detoxified reaction product in a reaction vessel having a heat exchange conduit extending therethrough. The process comprises the steps of loading the cyanide-containing waste into the reaction vessel, heating heat exchange fluid in a heating vessel to a temperature sufficient to initiate hydrolysis of cyanide in the cyanide-containing waste, circulating the hot heat exchange fluid between the heating vessel and the heat exchange conduit for a time sufficient to convert the cyanide-containing waste to the substantially detoxified reaction product, diverting the flow of heat exchange fluid from the heating vessel to a cooling apparatus, cooling the heat exchange fluid to a predetermined temperature by passage through the cooling apparatus, and circulating the cool heat exchange fluid between the cooling apparatus and the heat exchange conduit for a time sufficient to cool the substantially detoxified reaction product to a discharge temperature.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.